Recently, attention has been directed to a direct injection spark ignition type internal combustion engine, in which it is common to switchingly control the combustion mode corresponding to an engine driving condition, i.e., to switchingly control the combustion mode between: homogeneous combustion in which fuel is injected during intake stroke so that the same is diffused within a combustion chamber thereby forming a homogeneous air-fuel mixture; and stratified combustion in which fuel is injected during compression stroke so that a stratified air-fuel mixture is formed concentratedly about an ignition plug (see Japanese Unexamined Patent Publication No. 59-37236).
In such a switching control, it has been contemplated to provide a weakly stratified combustion region at a boundary region spanning between a homogeneous combustion region and a stratified combustion region in each of all the performance maps having an engine rotation speed and its torque as map parameters. In such a weakly stratified combustion region, there is conducted a two-step injection in which a part of fuel is injected during intake stroke and the remaining fuel is injected during compression stroke, so as to prevent such as: misfire and smoke-increase due to an over-rich state in the stratified combustion; and misfire and unstable combustion due to an over-lean state in the homogeneous combustion.
However, compared with a one-step injection (normal injection), the amount of fuel to be injected to the engine is divided in the two-step injection, so that the injection amount (injection pulse width) at one time in the two-step injection becomes smaller than that of the one-step injection.
As such, each of the divided injection amounts may become less than a minimum compensation amount (minimum compensation pulse width; a lowermost value which can compensate linearity in pulse width-flow rate characteristics of a fuel injection valve), so that the fuel injection valve can not realize the flow rate compensation, causing torque difference or smoking due to inclination toward lean or rich side.
The present invention has been carried out in view of the conventional problems as described above, and it is therefore an object of the present invention to control fuel injection amounts at the time of two-step injection, to thereby restrain occurrence of torque difference or smoking due to inclination toward lean or rich side, which would occur if either of fuel injection amounts was less than a minimum compensation amount.